I. Field
The present invention relates generally to communication, and more specifically to signal quality estimation in a wireless communication system.
II. Background
In a wireless communication system, a transmitter first digitally processes traffic/packet data to obtain coded data. The transmitter then modulates a carrier signal with the coded data to obtain a modulated signal that is more suitable for transmission via a wireless channel. The modulation may be performed based on various modulation schemes. One modulation scheme that is widely used throughout the world is continuous phase modulation (CPM). With CPM, the phase of the carrier signal is modulated by the coded data in a continuous rather than abrupt manner. As a result, CPM has several desirable characteristics such as (1) a constant envelope for the modulated signal, which allows the signal to be transmitted using an efficient power amplifier, and (2) a compact spectrum for the modulated signal, which enables efficient utilization of the available frequency spectrum.
A modulated signal generated with CPM (or simply, a CPM signal) has a fairly complex waveform that can complicate the design of a receiver used to process the CPM signal. To simplify the receiver design, the CPM signal may be approximated as a phase shift keying (PSK) modulated signal, as described below. This approximation for the CPM signal is sufficiently accurate for many applications and is often used for a CPM receiver.
A CPM receiver often needs to derive an estimate of the received signal quality. In general, signal quality (which is denoted as “C/I” herein) may be quantified by a carrier-to-interference ratio, a signal-to-noise ratio, a signal-to-noise-and-interference ratio, and so on. The C/I estimate may be used for various purposes such as, for example, to select an appropriate data rate for data transmission. The approximation of the CPM signal as a PSK modulated signal, while simplifying the receiver design, results in an inaccurate C/I estimate for certain operating conditions. The inaccurate C/I estimate can degrade system performance.
There is therefore a need in the art for techniques to derive a more accurate C/I estimate for a CPM signal.